1. Field of the Invention
The invention relates to a picture display device including a waveform generator for generating a periodical waveform for addressing picture elements of a display screen, said waveform being changeable by means of a plurality of adjusting parameters.
2. Description of Related Art
Such a waveform is useful for addressing picture elements of the display screen, for example, for addressing luminescing picture elements or for deflecting one or more electron beams in a picture display tube, either at the field frequency or at the line frequency. Field frequency waveforms are, for example, a sawtooth-shaped voltage for the field deflection, i.e. the deflection in the vertical direction, and a usually parabolic signal for the so-called east-west (raster) correction of the line deflection, i.e. the deflection in the horizontal direction. Other field frequency waveforms for correcting the deflection may be required for convergence or for dynamic focusing. Similarly, line frequency waveforms are generated for line deflection, convergence and focusing. For generating the various waveforms, the waveform generator receives a trigger signal which usually originates from a synchronizing circuit and which is substantially synchronous with a synchronizing signal present in an incoming video signal. As a result, the generated waveforms have the correct field or line frequency and substantially the same phase as the synchronizing signal. Circuit arrangements with which these signal processing operations can be performed are known. Such a circuit arrangement is described in, for example, "DIGIT 2000, VLSI Digital TV System" 1985, Rombach Druckhaus KG (Freiburg, DE), Chapter "DPU 2500, Deflection Processing Unit", pp. 111-134.
In this arrangement, each waveform to be generated depends on a plurality of parameters which must be adjusted. For example, for the sawtooth shape for the field deflection, there are usually four adjustable parameters, namely the DC component of the sawtooth shape, which determines the vertical position of the displayed picture on the display screen of the display tube, the amplitude, the symmetry with respect to the central horizontal line on the screen and the linearity correction. The relevant adjustments are not independent of one another. If, for example, the linearity is adjusted, it appears that the vertical position and the amplitude also change and that the displayed picture becomes asymmetrical with respect to the imaginary central horizontal line on the display screen. Similar problems occur when adjusting the other waveforms. If one of the parameters is adjusted, it should be repeatedly attempted to find new values for the other parameters until all parameters have a more or less correct value. Adjusting the parameters is therefore quite time-consuming.